Backlight Structures and Backlight Assemblies for Electronic Device Displays

ABSTRACT

An electronic device may have a liquid crystal display with backlight structures. The backlight structures may produce backlight that passes through the display layers in the display. The display layers may include a layer of liquid crystal material interposed between a color filter layer and a thin-film transistor layer. The backlight structures may include a light guide plate. A plurality of light-emitting diodes mounted on a flexible printed circuit may be coupled to an edge of the light guide plate. The flexible printed circuit may be curled into a spring element to bias the light-emitting diodes against the edge of the light guide plate. A plurality of gaps may be formed in the flexible printed circuit and may be used to separate and mechanically decouple adjacent light-emitting diodes. Individual light-emitting diodes may independently register to the light guide plate to maximize optical efficiency in the display.

This application is a continuation of U.S. patent application Ser. No.13/421,703, filed Mar. 15, 2012, which is hereby incorporated byreference herein in its entirety. This application claims the benefit ofand claims priority to U.S. patent application Ser. No. 13/421,703,filed Mar. 15, 2012.

BACKGROUND

This relates generally to electronic devices and, more particularly, toelectronic devices with displays and associated backlight structures.

Electronic devices such as computers and cellular telephones havedisplays. Some displays such as plasma displays and organiclight-emitting diode displays have arrays of display pixels thatgenerate light. In displays of this type, backlighting is not necessarybecause the display pixels themselves produce light. Other displayscontain passive display pixels that can alter the amount of light thatis transmitted through the display to display information for a user butdo not produce light themselves. As a result, it is often desirable toprovide backlight for a display with passive display pixels.

In a typical backlight assembly for a display, a light guide plate isused to distribute backlight generated by a light source such as alight-emitting diode light source. Optical films such as a diffuserlayer and brightness enhancing film may be placed on top of the lightguide plate. A reflector may be formed under the light guide plate toimprove backlight efficiency.

To provide satisfactory backlighting, it may be desirable to locate oneor more strips of light-emitting diodes on the edges of a light guideplate. A light strip of light-emitting diodes may be formed by mountinga row of light-emitting diodes onto a flex circuit. Light strips aretypically attached at the edges of the light guide plate so that thelight-emitting diodes can direct light into the light guide plate.

In an ideal light strip, the light-emitting diodes are aligned with eachother so that each light-emitting diode can physically contact the lightguide plate. However, there are often placement variations within a rowof light-emitting diodes that result in misalignment. If care is nottaken, this type of misalignment can result in air gaps between thelight-emitting diodes and the light guide plate. The presence of airgaps can have an adverse impact on backlight efficiency. Poor backlightefficiency may in turn decrease power consumption efficiency and canreduce battery life in an electronic device.

It would therefore be desirable to be able to provide electronic deviceswith improved arrangements for backlighting displays.

SUMMARY

A backlight assembly may be provided for producing backlightillumination for a display. The backlight assembly may have lightsources such as light-emitting diodes. The light-emitting diodes may beedge-emitting diodes that emit light through edges that areperpendicular to a base surface or may emit light through a surface thatopposes the base surface.

The backlight assembly may include a light guide plate. The light guideplate may have an upper surface through which backlight is provided tothe underside of the display. The light guide plate may also have edgeportions into which light may be launched from the light-emittingdiodes.

The light-emitting diodes may be mounted on a flexible substrate such asa flexible printed circuit formed form a flexible sheet of polymer. Theflexible printed circuit may be wrapped around a bend guiding structureto form a spring. The spring may press the light-emitting diodes againstthe edge portions of the light guide plate.

Slots or other decoupling features may be provided within the flexibleprinted circuit to mechanically decouple adjacent light-emitting diodesfrom each other. The slots may be rectangular in shape and may havelocally widened portions. Slots may be formed along one edge of aflexible printed circuit or may be formed on opposing edges of theflexible printed circuit so that the flexible printed circuit has aserpentine shape.

Perforations may be formed within the bent portion of a spring-shapedcurled flexible printed circuit. Locally widened traces may be formed onthe bent portion of a flexible printed circuit to enhance tracestrength.

A foam structure such as a thermally conductive foam that serves as aheat sink, a bent metal structure, or other biasing structure may beused to bias the flexible printed circuit and attached light-emittingdiodes against the edge portions of the light guide plate. The flexibleprinted circuit may be attached to a support structure using an adhesivethat allows the flexible printed circuit and the light-emitting diodesto laterally move relative to the edge portions of the light guideplate. This aligns the light-emitting diodes to the edge portions of thelight guide plate and minimizes gaps between the light-emitting diodesand the light guide plate. Rail holes within a flexible printed circuitmay be used to allow the flexible printed circuit and light-emittingdiodes to be laterally aligned with the light guide plate.

The light guide plate may have holes into which the light-emittingdiodes are placed. Index-of-refraction-matching material that matchesthe index-of-refraction of the light guide plate may be used to fillgaps between the light-emitting diodes and the light guide plate toimprove coupling efficiency. Reservoirs may be coupled to the holes toaccommodate excess index-of-refraction-material material.

Further features of the invention, its nature and various advantageswill be more apparent from the accompanying drawings and the followingdetailed description of the preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of an illustrative electronic device such as aportable computer having a backlit display in accordance with anembodiment of the present invention.

FIG. 2 is a diagram of an illustrative electronic device such as acellular telephone or other handheld device having a backlit display inaccordance with an embodiment of the present invention.

FIG. 3 is a diagram of an illustrative electronic device such as atablet computer having a backlit display in accordance with anembodiment of the present invention.

FIG. 4 is a diagram of an illustrative electronic device such as acomputer monitor with a built-in computer having a backlit display inaccordance with an embodiment of the present invention.

FIG. 5 is a cross-sectional side view of an illustrative backlit displayin accordance with an embodiment of the present invention.

FIG. 6 is a cross-sectional side view of a conventional backlightarrangement having air gaps between the light-emitting diodes and thelight guide plate.

FIG. 7 is a top view of a conventional backlight arrangement having airgaps between the light-emitting diodes and the light guide plate.

FIG. 8A is a cross-sectional side view of a backlight arrangement inwhich light-emitting diodes are mounted on a flexible substrate that iscurled into a spring element that biases the top of each light-emittingdiode against a light guide plate in accordance with an embodiment ofthe present invention.

FIG. 8B is a top view of a flexible substrate that may be used to formthe spring element of FIG. 8A in which the flexible substrate isprovided with gaps that separate and mechanically decouple adjacentlight-emitting diodes in accordance with an embodiment of the presentinvention.

FIG. 8C is a top view of a flexible substrate that may be used to formthe spring element of FIG. 8A in which the flexible substrate is free ofgaps in accordance with an embodiment of the present invention.

FIG. 9A is a cross-sectional side view of a backlight arrangement inwhich light-emitting diodes are mounted on a flexible substrate that iscurled into a spring element that biases the side of each light-emittingdiode against a light guide plate in accordance with an embodiment ofthe present invention.

FIG. 9B is a top view of a flexible substrate that may be used to formthe spring element of FIG. 9A in which the flexible substrate isprovided with gaps that separate and mechanically decouple adjacentlight-emitting diodes in accordance with an embodiment of the presentinvention.

FIG. 9C is a top view of a flexible substrate that may be used to formthe spring element of FIG. 9A in which the flexible substrate isprovided with locally widened gaps in accordance with an embodiment ofthe present invention.

FIG. 10A is a cross-sectional side view of a backlight arrangement inwhich a high shear adhesive is used to attach the flexible substrate toa support structure in accordance with an embodiment of the presentinvention.

FIG. 10B is a top view of a flexible substrate that may be used in thearrangement of FIG. 10A in which the flexible substrate has a serpentineshape that mechanically decouples adjacent light-emitting diodes inaccordance with an embodiment of the present invention.

FIG. 11A is a cross-sectional side view of a backlight arrangement inwhich a biasing structure is used to press light-emitting diodes againsta light guide plate in accordance with an embodiment of the presentinvention.

FIG. 11B is a top view of a flexible substrate that may be used in thearrangement of FIG. 11A in which the flexible substrate is provided withrail holes to attach the flexible substrate to a support structure inaccordance with an embodiment of the present invention.

FIG. 11C is a top view of a flexible substrate that may be used in thearrangement of FIG. 11A in which the flexible substrate has a serpentineshape that mechanically decouples adjacent light-emitting diodes inaccordance with an embodiment of the present invention.

FIG. 12A is a top view of a backlight arrangement in which anindex-matching material is used to fill gaps between light-emittingdiodes and a light guide plate in accordance with an embodiment of thepresent invention.

FIG. 12B is a cross-sectional side view of the backlight arrangement ofFIG. 12A in which an index-matching material is used to fill gapsbetween light-emitting diodes and a light guide plate in accordance withan embodiment of the present invention.

DETAILED DESCRIPTION

A display may be provided with backlight structures. The backlightstructures may produce backlight for the display that helps a user of adevice view images on the display in a variety of ambient lightingconditions. Displays with backlights may be provided in any suitabletype of electronic equipment.

An illustrative electronic device of the type that may be provided witha backlit display is shown in FIG. 1. Electronic device 10 may be acomputer such as a computer that is integrated into a display such as acomputer monitor, a laptop computer, a tablet computer, a somewhatsmaller portable device such as a wrist-watch device, pendant device, orother wearable or miniature device, a cellular telephone, a mediaplayer, a tablet computer, a gaming device, a navigation device, acomputer monitor, a television, or other electronic equipment.

As shown in FIG. 1, device 10 may include a backlit display such asdisplay 14. Display 14 may be a touch screen that incorporatescapacitive touch electrodes or other touch sensor components or may be adisplay that is not touch-sensitive. Display 14 may include image pixelsformed from liquid crystal display (LCD) components or other suitabledisplay pixel structures. Arrangements in which display 14 is formedusing liquid crystal display pixels are sometimes described herein as anexample. This is, however, merely illustrative. Any suitable type ofdisplay technology may be used in forming display 14 if desired.

Device 10 may have a housing such as housing 12. Housing 12, which maysometimes be referred to as a case, may be formed of plastic, glass,ceramics, fiber composites, metal (e.g., stainless steel, aluminum,etc.), other suitable materials, or a combination of any two or more ofthese materials.

Housing 12 may be formed using a unibody configuration in which some orall of housing 12 is machined or molded as a single structure or may beformed using multiple structures (e.g., an internal frame structure, oneor more structures that form exterior housing surfaces, etc.).

As shown in FIG. 1, housing 12 may have multiple parts. For example,housing 12 may have upper portion 12A and lower portion 12B. Upperportion 12A may be coupled to lower portion 12B using a hinge thatallows portion 12A to rotate about rotational axis 16 relative toportion 12B. A keyboard such as keyboard 18 and a touch pad such astouch pad 20 may be mounted in housing portion 12B.

In the example of FIG. 2, device 10 has been implemented using a housingthat is sufficiently small to fit within a user's hand (i.e., device 10of FIG. 2 may be a handheld electronic device such as a cellulartelephone). As show in FIG. 2, device 10 may include a backlit displaysuch as display 14 mounted on the front of housing 12. Display 14 may besubstantially filled with active display pixels or may have an inactiveportion and an inactive portion. Display 14 may have openings (e.g.,openings in the inactive or active portions of display 14) such as anopening to accommodate button 22 and an opening to accommodate speakerport 24.

FIG. 3 is a perspective view of electronic device 10 in a configurationin which electronic device 10 has been implemented in the form of atablet computer. As shown in FIG. 3, backlit display 14 may be mountedon the upper (front) surface of housing 12. An opening may be formed indisplay 14 to accommodate button 22.

FIG. 4 is a perspective view of electronic device 10 in a configurationin which electronic device 10 has been implemented in the form of acomputer integrated into a computer monitor. As shown in FIG. 4, backlitdisplay 14 may be mounted on the front surface of housing 12. Stand 26may be used to support housing 12.

A cross-sectional side view of display 14 is shown in FIG. 5. As shownin FIG. 5, display 14 may include backlight structures 30. Backlightstructures 30 may include a light source such as light-emitting diodelight source 38, a light guide plate such as light guide plate 32,optical films 34, and a reflector such as reflector 36. Duringoperation, light-emitting diode light source 38 may emit light 44 intolight guide plate 32. Light guide plate 32 may be formed from arectangular sheet of clear plastic. Light 44 may travel within lightguide plate 32 by means of total internal reflection. Light that escapesthrough the upper surface of light guide plate 32 may pass throughoverlying display layers in direction z and may serve as backlight fordisplay 14. Light that escapes through the lower surface of light guideplate 32 may be reflected by reflector 36 and redirected upwards indirection z. Reflector 36 may be formed from a reflective material suchas white plastic (as an example). Optical films 34 may includebrightness enhancing film layers, diffusing film layers, andcompensating film layers (as examples).

Display 14 may have an upper polarizer layer such as polarizer layer 40and a lower polarizer layer such as polarizer layer 42. Polarizer layer42 may polarize backlight 44. Thin-film transistor (TFT) layer 46 mayinclude a layer of thin-film transistor circuitry and an array ofassociated pixel electrodes. Pixel structures such as thin-filmtransistor structures and associated pixel electrodes in the array ofpixel electrodes on thin-film transistor layer 46 may produce electricfields corresponding to image data to be displayed. The electric fieldproduced by each electrode on thin-film transistor layer 46 adjusts theorientation of liquid crystals in an associated portion of liquidcrystal layer 48 by a corresponding amount. As light travels throughdisplay 14, the adjustment of the orientation of the liquid crystalsadjusts the polarization of the light that passes through layer 48. Whenthis light reaches upper polarizer 40, the polarization state of eachpixel of light is attenuated by an amount that is proportional to itspolarization, thereby creating visible images for a user.

Color filter layer 50 may contain an array of colored pixels (e.g., anarray of red, blue, and green color filter elements) for providingdisplay 14 with the ability to form color images. Sealant 52 may be usedto seal color filter layer 50 to thin-film transistor layer 46 and toretain liquid crystal material 48 within display 14.

Display 14 may include a touch-sensitive layer such as touch-sensitivelayer 54 for receiving touch input from a user of device 10.Touch-sensitive layer 54 may include a pattern of indium tin oxide (ITO)electrodes or other suitable transparent electrodes that have beendeposited to form a capacitive touch sensor array. Touch-sensitive layer54 may, in general, be configured to detect the location of one or moretouches or near touches on touch-sensitive layer 54 based on capacitive,resistive, optical, acoustic, inductive, or mechanical measurements, orany phenomena that can be measured with respect to the occurrence of theone or more touches or near touches in proximity to touch-sensitivelayer 54. If desired, touch-sensitive layer 54 may be incorporated intothin-film transistor layer 46 (e.g., display pixel electrodes andcapacitive touch electrodes may be formed on a common substrate). Theexample of FIG. 5 in which touch-sensitive layer 54 is separate fromthin-film transistor layer 46 is merely illustrative.

If desired, additional layers may be included in display 14. An optionallayer of transparent glass or plastic may be used to provide aprotective cover for display 14, as illustrated by cover layer 56 ofFIG. 5.

A cross-sectional side view of a conventional backlight arrangement isshown in FIG. 6. As shown in FIG. 6, backlight illumination is providedby a strip of light-emitting diodes 202 located along the edge of lightguide plate 204. Light-emitting diodes 202 are mounted to flexibleprinted circuit substrate 206, typically using solder. Due to placementvariation during the mounting process, light-emitting diodes 202 areoften misaligned. As a result, an air gap G forms between light-emittingdiodes 202 and the edge of light guide plate 204. Such air gaps G canhave an adverse impact on backlight efficiency. For example, light 208will experience a change in refractive index as it travels from air gapG to light guide plate 204. This in turn will alter the angle at whichlight 208 enters light guide plate 204, possibly inhibiting the abilityof light guide plate 204 to evenly distribute backlight to the entiredisplay.

A top view of a conventional backlight arrangement with air gaps isshown in FIG. 7. As shown in FIG. 7, light-emitting diodes 202 aremisaligned, resulting in air gaps G between light-emitting diodes 202and light guide plate 204. Because light-emitting diodes 202 aremechanically coupled together by the solid strip of flexible printedcircuit substrate 206, individual light-emitting diodes 202 are not ableto independently register to light guide plate 204. As a result,backlight structures 200 will have low optical efficiency.

FIG. 8A is a cross-sectional side view of a portion of display 14illustrating how the optical efficiency of display 14 may be maximized.As shown in FIG. 8A, backlight structures 30 may include light guideplate 32, reflector 36, and a plurality of light-emitting diodes 38.Light-emitting diodes 38 may be mounted on a strip of flexible printedcircuit (sometimes referred to as a “flex circuit” or “flex tail”) suchas flex circuit 60. Flex circuit 60 and other flexible printed circuitsin device 10 may be formed from sheets of polyimide and/or other layersof polymer. Flex circuit 60 may include patterned metal traces to whichpackaged light-emitting diodes 38 are soldered. Patterned metal traceson flex circuit 60 may be used to distribute power to conductiveterminals of light-emitting diodes 38. The strip of flex circuit 60 onwhich the plurality of light-emitting diodes 38 is mounted is sometimesreferred to as a “light strip” or a “light bar.”

Backlight structures 30 may be mounted within an optional supportstructure such as support structure 62. Support structure 62 (sometimesreferred to as a chassis or mechanical chassis) may be formed frommaterials such as plastic, ceramic, fiber composites, metal, or othersuitable materials. If desired, display 14 may be formed by mountingbacklight structures 30 directly within housing 12 or by mountingbacklight structures 30 in support structures of other shapes. In theillustrative configuration of FIG. 8A, mechanical chassis 62 is used informing a backlight assembly for display 14 that may be mounted withinhousing 12 under a display cover layer such as display cover layer 56 ofFIG. 5. Other mounting configurations may be used, if desired.

As shown in FIG. 8A, light-emitting diodes 38 may be interposed betweenflex tail 60 and light guide plate 32. Each light-emitting diode 38 mayhave a base surface that is mounted (e.g., soldered) to flex tail 60 anda top surface 38T opposing the base surface that emits light into lightguide plate 32. To ensure that light-emitting diodes 38 press againstlight guide plate 32, flex tail 60 may be curled and/or bent to form aspring element such as spring element 60P. Spring element 60P may exerta force on light-emitting diodes 38 in direction 66 (e.g., towards lightguide plate 32). To form flex tail 60 into this type of spring element,flex tail 60 may be curled inside of support structure 62. In an attemptto return to its equilibrium position (e.g., uncurled), flex tail 60will naturally exert a force in direction 66, thereby pressing topsurface 38T of light-emitting diodes 38 against light guide plate 32.Light may be emitted from top surface 38T of light-emitting diode 38directly into the edge of light guide plate 32. Forming flex tail 60into a spring element that biases light-emitting diodes 38 against lightguide plate 32 may help reduce or eliminate air gaps betweenlight-emitting diodes 38 and light guide plate 32.

If desired, a bend-guiding structure such as bend-guiding structure 35may optionally be used to form and shape flex tail 60 into springelement 60P. Flex tail 60 may be bent around bend-guiding structure 35to form the desired bend in flex tail 60. Bend-guiding structure 35(sometimes referred to as a mandrel) may be a compliant or undersizedstructure and may be formed from materials such as foam, rubber,plastic, or other suitable materials. For example, bend-guidingstructure 35 may have an elongated rod shape that runs parallel to anedge of electronic device 10. A curved surface on bend-guiding structure35 may be used in forming a bent portion on flex tail 60 as flex tail 60curls around bend-guiding structure 35. If desired, bend-guidingstructure 35 may be heated while manipulating flex tail 60 into springelement 60P. Bend-guiding structure 35 may be formed as an integral partof housing 12, may be formed as an integral part of support structure62, or may be a separate structure used to form flex tail 60 into springelement 60P.

Some areas such as bend region 61 of spring element 60P may have asmaller bend radius than other areas of spring element 60P. Measures maybe taken to minimize the stress on flex tail 60 in regions such asregion 61. For example, region 61 of flex tail 60 may be provided withperforations, may be preformed (e.g., using a heated forming process ora cold forming process), may have reduced layers (e.g., copper platingin the bend region of flex circuit 60 may be reduced to one layer toincrease flexibility in the bend region), etc. Patterned traces may bestrengthened in portions of flex tail 60 that have a small bend radiusby increasing the width of the traces in the bend region. Light-emittingdiodes 38 may be mechanically decoupled from one another so that eachindividual light-emitting diode 38 may independently register to lightguide plate 32. To decouple light-emitting diodes 38, gaps may be formedin flex tail 60 to separate adjacent light-emitting diodes 38. A topview of flex tail 60 in which gaps are used to separate adjacentlight-emitting diodes 38 is shown in FIG. 8B. If desired, flex tail 60of FIG. 8B may be used to form the spring element of FIG. 8A.

As shown in FIG. 8B, a plurality of gaps such as gaps 68 may be formedin flex tail 60, thereby creating a plurality of separated flexible“tabs.” Each light-emitting diode 38 may be mounted on an associatedflexible tab. Gaps (sometimes referred to as slots) may separate andmechanically decouple light-emitting diodes 38 from one another,allowing each to independently register to light guide plate 32. Ifdesired, traces such as trace 67 may be locally widened in the bentportions of flex tail 60 to enhance the strength of the traces in thebent portions.

In FIG. 8B, flex tail 60 is shown in a flat position (e.g., “uncurled”).When flex tail 60 is curled into the shape of spring element 60P shownin FIG. 8A, top surface 38T of each light-emitting diode 38 will bepressed against the edge of light guide plate 32. The force provided byspring element 60P may push top surfaces 38T of light-emitting diodes 38up against light guide plate 32, and each individual light-emittingdiode 38 may independently register to light guide plate 32.

In the example of FIG. 8B, gaps 68 are used to isolate each individuallight-emitting diode 38 (e.g., a single light-emitting diode 38 ismounted on each flexible tab). This is, however, merely illustrative. Ifdesired, gaps 68 may be used to isolate groups of light-emitting diodes38. For example, there may be two, three, or more than threelight-emitting diodes on an associated flexible tab, if desired. Ingeneral, any number of gaps 68 may be used to separate any number oflight-emitting diodes 38.

FIG. 8C is a top view of another possible configuration of flex tail 60that may be used to form spring element 60P of FIG. 8A. As shown in FIG.8C, flex tail 60 on which light-emitting diodes 38 are mounted may be asolid strip of flexible printed circuit substrate. In FIG. 8C, flex tail60 is shown in a flat position (e.g., “uncurled”). When flex tail 60 iscurled into the shape of spring element 60P shown in FIG. 8A, the forceprovided by spring element 60P will push top surface 38T of eachlight-emitting diode 38 up against light guide plate 32. Light may beemitted from top surface 38T of light-emitting diodes 38 directly intothe edge of light guide plate 32.

Since top surface 38T of each light-emitting diode 38 registers to lightguide plate 32, any placement variation in light-emitting diodes 38 onflex tail 60 (e.g., variation in location on the surface of flex tail60) will not affect the light-emitting diodes' ability to physicallycontact light guide plate 32. The force provided by spring element 60Pwill push top surface 38T in direct contact with light guide plate 32regardless of any lateral misalignment on the surface of flex tail 60.

FIG. 9A is a cross-sectional side view of a portion of display 14illustrating another possible backlight assembly that may be used tooptimize the optical efficiency of display 14. As shown in FIG. 9A, flextail 60 may be curled and/or bent to form spring element 60P. Springelement 60P of FIG. 9A may exert a force on light-emitting diodes 38 indirection 72 (e.g., towards light guide plate 32), thereby pressing sidesurface 38S of light-emitting diodes 38 against light guide plate 32.Light may be emitted from side surface 38S of each light-emitting diode38 (e.g., a surface that is perpendicular to the base surface oflight-emitting diode 38) directly into the edge of light guide plate 32.

If desired, bend-guiding structure 35 may optionally be used to form andshape flex tail 60 into spring element 60P. Flex tail 60 may be wrappedaround bend-guiding structure 35 to form the desired bend in flex tail60. To minimize the stress on flex tail 60 in areas of small bendradius, portions of flex tail 60 may be perforated, may be preformed(e.g., using a heated forming process or a cold forming process), mayhave reduced layers (e.g., copper plating in bend regions of flexcircuit 60 may be reduced to one layer to increase flexibility in thebend region), etc. Patterned traces may be strengthened in portions offlex tail 60 that have a small bend radius by increasing the width ofthe traces in the bend region.

If desired, light-emitting diodes 38 may be mechanically decoupled fromone another so that each individual light-emitting diode 38 mayindependently register to light guide plate 32. A top view of flex tail60 in which gaps are used to separate adjacent light-emitting diodes 38is shown in FIG. 9B. If desired, flex tail 60 of FIG. 9B may be used toform the spring element of FIG. 9A.

As shown in FIG. 9B, a plurality of gaps 68 (e.g., rectangular slots)may be formed in flex tail 60, thereby creating a plurality of separatedflexible tabs. Each light-emitting diode 38 may be mounted on anassociated flexible tab. Gaps 68 may separate and mechanically decouplelight-emitting diodes 38 from one another, allowing each toindependently register to light guide plate 32.

In FIG. 9B, flex tail 60 is shown in a flat position (e.g., “uncurled”).When flex tail 60 is curled into the shape of spring element 60P shownin FIG. 9A, side surface 38S of light-emitting diode 38 may be in directcontact with light guide plate 32. The force provided by spring element60P may push side surface 38S of each light-emitting diode 38 againstlight guide plate 32. Because light-emitting diodes 38 are mechanicallydecoupled from one another, misalignment in light-emitting diodes 38will not affect the ability of individual light-emitting diodes 38 tophysically contact the edge of light guide plate 32.

During manufacturing, a light-emitting diode may be unintentionallysoldered to a flex tail at a slight angle. If care is not taken, thistype of angled position may lead to an air gap between thelight-emitting diode and the light guide plate. To ensure thatlight-emitting diodes 38 are flush with the edge of light guide plate32, light-emitting diodes 38 may be provided with rotationalcapabilities. FIG. 9C is a top view of flex tail 60 in whichlight-emitting diodes 38 are provided with rotational capabilities. Ifdesired, flex tail 60 of FIG. 9C may be used to form spring element 60Pof FIG. 9A.

As shown in FIG. 9C, a plurality of slots 68 may be interposed betweenadjacent light-emitting diodes 38. Slots 68 may have locally widenedportions such as widened portions 74. Having slots with locally widenedportions may allow each light-emitting diode 38 to rotate slightly, asindicated by arrows 76 in FIG. 9C. Providing light-emitting diodes 38with rotational capabilities may ensure that the entire side surface 38Sof each light-emitting diode 38 is in direct contact with light guideplate 32.

FIG. 10A is a cross-sectional side view of a portion of display 14illustrating another possible backlight arrangement that may optimizethe optical efficiency of display 14. As shown in FIG. 10A, flex tail 60may lie flat along the edge of light guide plate 32.

A layer of adhesive such as adhesive 82 may be interposed between flextail 60 and support structure 62. Adhesive 82 may be a high shearadhesive that attaches flex tail 60 to support structure 62 whileallowing some movement of flex tail 60 along the surface of supportstructure 62. A high shear adhesive such as adhesive 82 may provide ameans of securing flex tail 60 to the interior of support structure 62without constricting its lateral movement on the surface of supportstructure 62 (e.g., without inhibiting registration between light guideplate 32 and light-emitting diodes 38). Adhesive 82 may be formed frompressure sensitive adhesive, UV-curable adhesive, air-curable adhesive,moisture-curable adhesive, or other suitable type of adhesive. Ifdesired, adhesive 82 may be used as a heat sink. For example, adhesive82 may be formed from a material with high thermal conductivity and maybe configured to transfer heat from backlight structures 30 to supportstructure 62, housing 12, or other suitable heat spreader in thevicinity of backlight structures 30.

The plurality of light-emitting diodes 38 that are mounted on flex tail60 may be mechanically decoupled from one another so that eachindividual light-emitting diode 38 may independently register to lightguide plate 32. A top view of a flex tail of the type that may be usedin the configuration of FIG. 10A is shown in FIG. 10B.

As shown in FIG. 10B, flex tail 60 may have a serpentine shape in whichgaps are formed on both sides of flex tail 60. For example, a gap suchas gap 68A may be formed on side A of flex tail 60, between adjacentlight-emitting diodes 38. The next closest gap such as gap 68B may beformed on side B of flex tail 60 (e.g., the opposite side of flex tail60). The gaps may alternate sides along the length of flex tail 60 tocreate a serpentine-shaped flexible substrate.

When backlight structures 30 are inserted into support structure 62A, aforce may be applied in direction 84 (FIG. 10A). This may push flex tail60 into rear wall 62R of support structure 62 and reduce or eliminategaps between light-emitting diodes 38 and light guide plate 32. Due tothe serpentine-shape of flex tail 60 (FIG. 10B), light-emitting diodes38 may be mechanically decoupled from one another so that side surface38S of each light-emitting diode 38 may independently register to lightguide plate 32.

FIG. 11A is a cross-sectional side view of a portion of display 14illustrating another possible backlight assembly that may be used tooptimize the optical efficiency of display 14. As shown in FIG. 11A,flex tail 60 may lie flat along the edge of light guide plate 32. Abiasing structure such as biasing structure 92 may be used to biaslight-emitting diodes 38 against light guide plate 32 to help reduce oreliminate air gaps between light-emitting diodes 38 and light guideplate 32. Biasing structure 92 may be interposed between light-emittingdiodes 38 and rear wall 62R of support structure 62.

If desired, biasing structure 92 may be formed from a conformable,thermally conductive foam (e.g., a foam formed from Gap Pad® material orother suitable material). Using a thermally conductive material to formbiasing structure 92 may allow biasing structure 92 to transfer heatfrom backlight structures 30 (e.g., from light-emitting diodes 38) tosupport structure 62, housing 12, or other suitable heat spreader in thevicinity of backlight structures 30. Other structures that may be usedto bias light-emitting diodes 38 against light guide plate 32 includemetal-filled foam, a V-shaped structure (e.g., a V-shaped metal springmember), a spring structure, other suitable structures, etc.

If desired, an optional adhesive such as high shear adhesive 82 may beused to attach flex tail 60 to support structure 62 without constrictingits lateral movement on the surface of support structure 62 (e.g.,without resisting the biasing force provided by biasing structure 92).This is, however, merely illustrative. Other methods may be used toattach flex circuit 60 to support structure 62. For example, opposingends of flex tail 60 may be provided with rail holes. Screws or pins maybe used to secure flex tail 60 to support structure 62 at the railholes. FIG. 11B is a top view of flex tail 60 illustrating how railholes may be used to attach flex tail 60 to support structure 62.

As shown in FIG. 11B, holes such as rail holes 98 (sometimes referred toas openings or slots) may be formed in opposing ends of flex tail 60. Apin such as pin 96 may be inserted through each rail hole 98. Pins 96may be used to fasten flex tail 60 to support structure 62. Pins 96 maybe mushroom pins, straight pins, or any other suitable type of pin orscrew. Rail holes 98 may have an elongated “rail” shape. Movement offlex tail 60 in the y and z-directions may be restricted, whereasmovement in the x-direction may be permitted (e.g., movement along thelength of elongated rail holes 98 may be permitted). This type offastening method may provide a means of securing flex tail 60 to theinterior of support structure 62 without constricting its lateralmovement relative to light guide plate 32 (e.g., without resisting thebiasing force provided by biasing structure 92).

When backlight structures 30 are inserted into support structure 62,biasing structure 92 may exert a force in direction 94 (e.g., biasingstructure 92 may bias light-emitting diodes 38 against light guide plate32). The biasing force provided by biasing structure 92 may reduce oreliminate gaps between light-emitting diodes 38 and light guide plate32. If desired, flex tail 60 may be formed with a serpentine-shape asshown in FIG. 11C. The serpentine-shape of flex tail 60 may be used tomechanically decouple light-emitting diodes 38 from one another so thatside surface 38S of each light-emitting diode 38 may independentlyregister to light guide plate 32.

FIG. 12A is a top view of a portion of display 14 illustrating anotherpossible backlight assembly that may be used to optimize the opticalefficiency of display 14. As shown in FIG. 12A, light guide plate 32 mayhave a row of holes 32P that extends parallel to one of the edges oflight guide plate 32. Each hole 32P (sometimes referred to as an openingor light guide plate opening) may be enclosed and surrounded by lightguide plate material.

Backlight structures 30 may include a row of light-emitting diodes 38mounted on flex tail 60. An edge of light guide plate 32 may overlapflex tail 60 such that the row of light-emitting diodes 38 aligns withthe row of light guide plate openings 32P. Light-emitting diodes 38 mayeach be positioned within an associated hole 32P. Holes 32P may have anysuitable shape that accommodates light-emitting diodes 38 whenlight-emitting diodes 38 are inserted into light guide plate openings32P. If desired, more than one light-emitting diode 38 may be mountedinto an associated light guide plate opening 32P. The example of FIG.12A in which a single light-emitting diode 38 is mounted in each opening32P is merely illustrative.

To eliminate air gaps between light-emitting diodes 38 and light guideplate 32, an index-matching material such asindex-of-refraction-matching material 104 may be used to fill holes 32Paround each light-emitting diode 38. The refractive index ofindex-matching material 104 may be matched to the refractive index oflight guide plate 32. In this type of configuration, the angle at whichlight from light-emitting diode 38 enters light guide plate 32 will notbe effected by a change in refractive index as it passes fromindex-matching material 104 to light guide plate 32. Index-matchingmaterial 104 may be optically clear and may be formed from UV-curableadhesive, air-curable adhesive, moisture-curable adhesive, gel, or othersuitable materials.

If desired, an optional reservoir such as reservoir 102 may be formed inlight guide plate 32 adjacent to an associated opening 32P. Reservoir102 may be formed as an extension to opening 32P (e.g., a recess orcavity adjacent to opening 32P). Reservoir 102 may be configured toreceive excess index-matching material 104 in opening 32P. Reservoir 102may be formed on a side of light-emitting diode 38 that does not emitlight (e.g., a rear side of light-emitting diode 38 as shown in FIG.12A). If desired, light guide plate openings 32P may not be providedwith reservoirs 102. The example of FIG. 12A in which light guide plateopenings 32P are provided with reservoirs 102 for receiving excessindex-matching material 104 is merely illustrative.

A cross-section of backlight structures 30 taken along axis 106 is shownin FIG. 12B. As shown in FIG. 12B, light 44 may be emitted fromlight-emitting diode 38 and may travel through index-matching material104 and into light guide plate 32. Index-matching material 104 mayensure that the angle at which light enters light guide plate 32 is notaffected by a change refractive index at interface 108. Light 44 maytravel within light guide plate 32 by means of total internalreflection. Light that escapes through the upper surface of light guideplate 32 may pass through overlying display layers in direction z andmay serve as backlight for display 14. Light that escapes through thelower surface of light guide plate 32 may be reflected by reflector 36and redirected upwards in direction z.

If desired, light may be launched into light guide plate 32 from morethan one edge of plate 32. For example, a strip of light-emitting diodes38 may be placed along one edge, two edges, three edges, or all fouredges of light guide plate 32. The example of FIG. 12A in whichlight-emitting diodes 38 are located along one edge of light guide plate32 is merely illustrative.

The foregoing is merely illustrative of the principles of this inventionand various modifications can be made by those skilled in the artwithout departing from the scope and spirit of the invention. Theforegoing embodiments may be implemented individually or in anycombination.

What is claimed is:
 1. An electronic device, comprising: a transparentmember having a planar surface; a flexible printed circuit that is bentto form a spring element; and an electrical component mounted to theflexible printed circuit, wherein the spring element formed by theflexible printed circuit biases the electrical component against theplanar surface.
 2. The electronic device defined in claim 1 wherein theflexible printed circuit comprises first and second opposing ends,wherein the electrical component overlaps both the first end and thesecond end of the flexible printed circuit.
 3. The electronic devicedefined in claim 1 wherein the flexible printed circuit comprisesconductive traces and wherein the conductive traces are locally widenedin regions of the flexible printed circuit that are bent.
 4. Theelectronic device defined in claim 1 wherein the electrical componentcomprises opposing upper and lower surfaces, wherein the lower surfaceis mounted to the flexible printed circuit and wherein the upper surfaceis pressed against the transparent member.
 5. The electronic devicedefined in claim 1 wherein the electrical component comprises first andsecond surfaces, wherein the first surface is mounted to the flexibleprinted circuit, wherein the second surface is pressed against thesurface of the transparent member, and wherein the first surface of theelectrical component and the surface of the transparent member areperpendicular to each other.
 6. The electronic device defined in claim 1wherein the electrical component comprises an optical component andwherein light is conveyed between the transparent member and theelectrical component via the planar surface.
 7. The electronic devicedefined in claim 1 wherein the electrical component comprises alight-emitting diode.
 8. The electronic device defined in claim 1wherein the transparent member comprises a transparent sheet of plastic.9. The electronic device defined in claim 1 wherein the transparentmember comprises a light guide plate.
 10. An electronic device,comprising: a transparent planar structure having a surface; a flexibleprinted circuit curled to form a spring element; and an electricalcomponent mounted to the flexible printed circuit, wherein the springelement formed by the flexible printed circuit presses the electricalcomponent against the surface and wherein light is conveyed between theelectrical component and the transparent planar structure via thesurface.
 11. The electronic device defined in claim 10 wherein theelectrical component comprises a light-emitting diode.
 12. Theelectronic device defined in claim 11 further comprising an additionallight-emitting diode mounted to the flexible printed circuit, whereinthe flexible printed circuit comprises a slot interposed between thelight-emitting diode and the additional light-emitting diode.
 13. Theelectronic device defined in claim 10 wherein the flexible printedcircuit comprises first and second opposing ends, wherein the electricalcomponent is mounted to the first end, and wherein the first end isinterposed between the electrical component and the second end.
 14. Theelectronic device defined in claim 10 wherein the transparent planarstructure forms part of a display in the electronic device.
 15. Theelectronic device defined in claim 10 wherein the flexible printedcircuit has a serpentine shape.
 16. Apparatus, comprising: a planarmember having a surface; a flexible printed circuit that is bent to forma spring; and an electrical component mounted to the flexible printedcircuit, wherein the spring formed by the flexible printed circuitbiases the electrical component against the surface of the planarmember.
 17. The apparatus defined in claim 16 wherein the planar memberis transparent and wherein light is conveyed between the electricalcomponent and the planar member via the surface.
 18. The apparatusdefined in claim 17 wherein the electrical component comprises a lightsource.
 19. The apparatus defined in claim 16 wherein the flexibleprinted circuit comprises a bend region between a first portion of theflexible printed circuit and a second portion of the flexible printedcircuit and wherein the first and second portions overlap and areparallel to each other.
 20. The apparatus defined in claim 16 whereinthe electrical component comprises upper and lower opposing surfaces,wherein the lower surface is mounted to an end portion of the flexibleprinted circuit, and wherein the upper surface is pressed against thesurface of the planar member.